1. Field of the Invention
The present invention relates to a method of focusing an electron beam on a specimen in an automatic way and is particularly, but not exclusively, concerned with the application of such an automatic focussing method to an electron microscope. The present invention also relates to a device for focussing an electron beam, to a method of operating an electron microscope, and to an electron microscope having an automatic focussing device.
2. Summary of the Prior Art
It is known to provide automatic focussing of the electron beam of an electron microscope. Standard automatic focussing methods involve investigation of the optimum focus for a fixed magnification, which fixed magnification is determined by the operator, and which normally corresponds to the magnification at which the operator wishes to investigate a specimen. In such a focussing method, the determination of the optimum focus at the fixed magnification is carried out on the basis of detection of secondary electrons emitted from the specimen when the electron beam of the electron microscope is incident on the specimen. A suitable detector can detect such secondary electrons, and by investigating the variation in the secondary electrons as the electron beam is scanned across the specimen, the optimum focus can be determined. In order to do this, the variation in the secondary electron signal is investigated for each of a plurality of current values, which current values are the values of the current controlling the electromagnetic condenser lens which determines the focussing of the electron beam. This way, a range of outputs is obtained, and the optimum one of those values can be selected.
In U.S. Pat. No. 4,199,681 it is proposed that an automatic scanning method be provided in which there is variation in the magnification. First, the optimum output of secondary electrons is investigated for a relatively low magnification. At that low magnification, the current to the condenser lens is varied stepwise, with a first interval between the steps, until the optimum value has been exceeded. Then, the magnification is increased, and the focussing current is again varied stepwise, in the opposite direction to the first variation, and with a second interval between the steps, which second interval is smaller than the first. Again, once the optimum output has been passed, the magnification is changed, the direction of the stepwise variation is changed, and the interval of that variation is again decreased. In this way, it is intended that the system will focus correctly.